Various means are currently available to separate the components of a multicomponent fluid mixture. If the densities of the components differ sufficiently, the effects of gravity over time may be adequate to separate the components. Depending on the quantities of the components involved, a centrifuge may be used to more rapidly separate components with different densities. Alternatively, distillation may be used to separate components with different boiling points.
Some fluid mixtures comprise components which have similar boiling points, and in such cases, separation by distillation may be a difficult and an inefficient means to separate these components. Too many contaminants, e.g., unwanted components, also may evaporate along with (or fail to evaporate from) the desired component(s), or the separation may require high energy expenditures due to the recycling through the distillation process that may be necessary to attain a desired degree of separation or purity.
In view of these and other deficiencies of these aforementioned processes, adsorption often has been preferred as a process for separating the components from a multicomponent fluid mixture to obtain relatively pure products.
The efficiency of an adsorption process may be partially dependent upon the amount of the surface area of the adsorbent solids which is available for contact with a fluid mixture. The surface area available may be more than just the superficial, external surface of the solids. Suitable solids also may have internal spaces. Such internal spaces may comprise pores, channels, or holes in the surface of the solids and may run throughout the solids, much as in sponges. Thus, the fluid contacts not only the superficial surface, but penetrates into the solids. Internal spaces increase the contact surface between the fluid and the solids in an adsorption process by concentrating them in a confined space. Examples of solids with internal void spaces include solids described as molecular sieves. The volumetric amount of components that may be adsorbed by a molecular sieve is termed the molecular sieve capacity.
In an adsorption process, separation of the fluid components may be accomplished because the adsorbent solid material may have a physical attraction for one or more of the components of the mixture in preference to other components of the mixture. Although all of the components of a mixture may be attracted in varying degrees to the material, there is a preference engineered into the process, such that predominantly the desired component(s) may be attracted and remain with the material in preference over all others. Therefore, even if less preferred components of a mixture initially come into contact with a portion of the material, because of the stronger attraction of the material for the desired component(s) of the mixture, the less preferred component(s) may be displaced from the material by the desired, and more strongly preferred, component(s). Although the fluid mixture entering an adsorbent bed might be composed of multiple components, the fluid mixture passed through the adsorbent bed would be depleted in the component(s) which are more preferentially adsorbed into the adsorbent. The concentration of the less preferentially adsorbed component(s), based upon the total concentration of more and less preferentially components, would be greater in the effluent from the bed than in the feed to the bed.
In adsorption processes using adsorbent solids, separation occurs for a period of time, but eventually all the available surface sites on and in the solids are taken up by the desired component(s) or are blocked by concentrations of unwanted components. At that point, little significant additional adsorption of component(s) from the mixture is likely to occur, and the fluid mixture which might be withdrawn from the chamber may be insignificantly changed by further exposure to the solids. The adsorption step of the process is thus ended, and the component(s) which have been adsorbed by the solids can then be removed from the solids, so as to effect separation and permit reuse of the solids.
A suitable adsorption apparatus or system might first permit adsorption of a product comprising the desired component(s) by the solids and later treat the solids to cause them to release the product and permit recovery of this product. Such an adsorption apparatus or system might comprise a “moving-bed” which permits movement of a tray or bed of the solids through a chamber, such that at different locations, the solid is subjected to different steps of an adsorption process, e.g., adsorption, purification, and desorption. These steps will be understood more clearly by the description below. Nevertheless, moving the solids through an adsorption apparatus may be difficult and involve complex machinery to move trays or beds. It also may result in loss of the solids by attrition. To avoid these problems, some adsorption apparatus and systems have been designed to “simulate” moving the tray(s) or bed(s) to the locations, e.g., zones, of different steps of an adsorption process. Simulation of the movement of the tray(s) or bed(s) may be accomplished by (1) maintaining a continuous, circulating flow of bulk fluid through the tray(s) or bed(s), while (2) varying, over time, the location of feed streams to the circulating bulk fluid, as well as the location of withdrawal streams from the circulating bulk fluid. The location of feed and withdrawal streams may be varied by use of a system of conduits which permits directing and redirecting the streams of fluids into the chamber to create different zones at different times. As these stream changes occur, the solids are eventually employed in different steps in an adsorption process as though the solids were moving in a countercurrent manner to the flow of the circulating bulk fluid through the chamber.
The different zones within an adsorption apparatus or system may be described by a particular step of the adsorption process performed within each zone, for example, (1) an adsorption step in an adsorption zone, (2) a purification step in a purification zone, and (3) a desorption step in the desorption zone. The bulk fluid, which circulates through the adsorption apparatus or system flows in a continuous sequence through the desorption zone, and then through the purification zone, and then through the adsorption zone. The simulated movement of the adsorbent beds occurs in the countercurrent direction of the flow of circulating bulk fluid. Thus, adsorbent beds, in a simulated manner, move in a continuous sequence, first through the adsorption zone, and then through the purification zone, and then through the desorption zone.
The circuit of bulk fluid flow is completed by passing circulating fluid from the purification zone to the adsorption zone and then to the desorption zone, et. seq. One or more buffer zones may be inserted between these zones, for example, between the adsorption zone and the desorption zone. A more detailed explanation of the zones of the adsorption process follows.
Adsorption Zone: A feed stream comprising C8 aromatics, e.g., orthoxylene (OX), metaxylene (MX), paraxylene (PX), and ethylbenzene (EB), is fed into the adsorption apparatus or system. The portion of the apparatus or system into which the feed stream is being fed and carried along with circulating fluid is termed an “adsorption zone.” The adsorption zone may comprise a plurality of beds of adsorbent material in a vessel. In the adsorption zone, the fluid comes into contact with the adsorbent material, and the desired component (PX) is adsorbed by the adsorbent material. As noted above, other components (MX, OX and EB) may also be adsorbed, but preferably to a lesser extent. This preferential adsorption may be achieved by the selection of an adsorbent material, e.g., an adsorbent solid, which has a preference for adsorbing the desired component (PX) from the multicomponent feedstream. Although only the desired component (PX) may have been adsorbed by the solids, other less preferentially adsorbed components (MX, OX and EB) of the fluid mixture may still remain in void spaces between the solids and possibly, in the pores, channels, or holes within the solids. The flow of the circulating bulk fluid through the adsorbent beds will tend to carry the unwanted components (MX, OX and EB) through the adsorbent material. These unwanted components (MX, OX and EB) preferably are removed from the solids before the desired component (PX) is recovered from the solids, so that they are not recovered along with the product.
Purification Zone: After adsorption, the next step is to purify or rectify the adsorbent beds, comprising adsorbed desired component (PX), in the adsorption chamber. In the literature, the purification zone is sometimes referred to as the rectification zone. In this step, beds of adsorbent material may be moved or the location of feed and effluent streams may be changed. For example, the feed point of the multicomponent feed stream may be moved from a first bed to a second bed located downstream from the first bed, in terms of the direction of flow of the circulating bulk fluid through the beds. Although the beds are not physically moved, the material may now be described as being in a “purification zone.” In this zone, the circulating bulk fluid is depleted of the preferentially adsorbed component in the feed stream. The circulating bulk fluid in the purification zone tends to dissolve and remove the unwanted components (MX, OX and EB) from the adsorbent material, e.g., from within and from the interstitial areas between the solids. Thus, a fluid comprising unwanted components, e.g., raffinate, passes through the purification zone along with the flow of the circulating bulk fluid. The unwanted components (MX, OX and EB) may be withdrawn in a raffinate stream located below the adsorption zone. Because an objective of the adsorption process is to separate the desired component (PX) from other components (MX, OX and EB), which have nearly the same boiling point or density as the desired component (PX), purification displaces unwanted components (MX, OX and EB) and substitutes another fluid (e.g., a desorbent) which can be more readily separated by other means, e.g., distilled.
Desorption Zone: After the adsorbent solids have, in a simulated manner (by virtue of changing the location of inlet and outlet streams of the adsorbent vessel), “passed through” the purification zone, the adsorbent solids enter the desorption zone. The desorption zone may comprise a plurality of adsorbent beds. A desorbent stream is introduced in one end of the desorption zone, along with the circulating bulk fluid to the adsorbent bed located furthest upstream in terms of the direction of flow of the circulating fluid through the beds of the desorption zone. The desorbent stream contains desorbent which is more preferentially adsorbed by the adsorbent solids than the product comprising the desired component (PX). The desorbent chosen will depend in part upon the desired component(s), the adsorbent materials, and the ease with which the desorbent can be separated from the product. The desorbent flows along with the circulating fluid and desorbs the desired component (PX) from the adsorbent solids. An extract stream is taken from the circulating fluid at the other end of the purification zone, which is located at the location of the adsorbent bed furthest downstream in terms of the direction of flow of the circulating fluid through the beds of the desorption zone. The extract stream may comprise the desired product (PX), desorbent and only trace or insignificant amounts of unwanted components (MX, OX and EB). Examples of desorbents include paradiethylbenzene (pDEB) and toluene (TOL).
Each and every step and zone might be present somewhere in an adsorption apparatus or system if simultaneous operations are conducted. Nevertheless, the steps may be performed successively or staggered over time. Further, in some adsorption processes, the unwanted components may be adsorbed, and the product comprising the desired component(s) allowed to pass through the adsorption apparatus or system. Therefore, in a given system, the terms raffinate and extract are relative and may depend upon the particular nature of the components being separated, the preference of the solids, and the nature of the apparatus or system.
An apparatus suitable for accomplishing the adsorption process of this invention is a simulated moving-bed adsorption apparatus. A commercial embodiment of a simulated moving-bed adsorption apparatus is used in the well-known Parex™ Process, which is used to separate C8 aromatic isomers and provide a more highly pure paraxylene (PX) from a less highly pure mixture. See by way of example U.S. Pat. Nos. 3,201,491; 3,761,533; and 4,029,717.
Such an adsorption apparatus may comprise at least one vertical column stacked with beds of adsorbent solids. The beds may be in trays packed with the adsorbent solids. One or more than one type of adsorbent solid may used. The column(s) may have the capability to perform each of the above-described steps simultaneously within different locations, e.g., zones, in the column(s). Thus, the composition of the fluid in the column(s) may vary between zones although there may be no structures completely separating these zones. A serially and circularly interconnected matrix of fluid communication conduits including associated valves, pumps, and so forth, may permit inlet and effluent streams to be directed and redirected into different zones of the column(s). The fluid communication conduits including associated valves, pumps, and so forth, may be configured to pass a variety of streams through each of the conduits. These streams may pass into the adsorbent vessel, as inlet streams, or out of the vessel as effluent streams. Over time, both inlet and effluent streams may pass through each of the individual conduits. The different zones within the chamber may have constantly shifting boundaries as the process is performed.
The circulating flow of bulk fluid through the adsorbent apparatus may be facilitated by pumping the effluent from the bottom bed of an adsorbent column and passing this effluent as an inlet stream to the top bed of another adsorbent column. When the adsorption process involves the use of more than one adsorbent columns connected in series, the effluent from the bottom bed of a first column may be passed as an inlet stream to the top bed of a second adsorbent column, and the effluent from the bottom bed of the last column in the series may be passed as an inlet stream to the top bed of the first adsorption column.
A manifold arrangement may be used to cause the adsorbent solids to flow, in a simulated manner, in a counter current manner with respect circulating bulk fluid. The valves in the manifold may be operated in a sequential manner to effect the shifting of inlet and outlet streams. In this regard, see U.S. Pat. No. 3,706,812. Another means for producing a simulated countercurrent flow of the solid adsorbent is a rotating disc valve by which inlet and outlet streams, e.g., feed, extract, desorbent, raffinate, and conduit flush, are cyclically changed during the course of the process. Both U.S. Pat. Nos. 3,040,777 and 3,422,848 disclose suitable rotary valves. Both suitable manifold arrangements and disc valves are known in the art. More recently, a system has been described using dual rotary valves. See U.S. Pat. No. 8,168,845.
Normally there are at least four streams (feed, desorbent, extract, and raffinate) employed in the procedure. The location at which the feed and desorbent streams enter a column of adsorbent beds and the extract and raffinate streams leave the column are simultaneously shifted in the same direction at set intervals. The direction of the shift is the same as the direction of the flow of the circulating bulk fluid through the adsorption chamber. Each shift in location of these transfer points delivers or removes liquid from a different bed within the column. In many instances, one zone may contain a larger quantity of adsorbent material than other zones. Moreover, zones other than those discussed above may also be present. For example, in some configurations, a buffer zone between the adsorption zone and the desorption zone may be present and may contain a small amount of adsorbent material relative to the zones surrounding it. Further, if a desorbent is used that can easily desorb extract from the adsorbent material, only a small amount of the material need be present in the desorption zone in comparison to the other zones. As noted above, the adsorbent need not be located in a single column, but may be located in multiple columns or a series of columns.
A plurality of fluid communication conduits may be used to introduce fluids to the beds and to withdraw fluids from the beds. The same fluid communication conduit may be used in a first instance to input a feedstream into the apparatus or system and later to withdraw an extract stream. This can result in reduced product purity due to contamination of the withdrawn product. Fluid communication conduits may contain unwanted components, such as residue remaining in the conduit from earlier additions or withdrawals of streams. This problem may be overcome by employing separate conduits for each stream or by removing such residue from the conduits by flushing them with a medium which would not affect product purity as adversely as would an unwanted component remaining in the fluid communication conduit. A preferred flushing medium has been the recycled product or the desorbent, which might be more readily separated downstream of the chamber than would the residue. See U.S. Pat. No. 4,031,156. Nevertheless, flushing conduits with the recycled product reduces the output of the adsorption process. Furthermore, the excessive use of desorbent may result in an increase in the desorbent consumption and may also desorb certain amount of sorbate adsorbed within the purification zone, thereby limiting the overall efficiency of the system.
A standard Parex™ unit for separating paraxylene (PX) from the other C8 aromatic isomers, metaxylene (MX), orthoxylene (OX), and ethylbenzene (EB), has a single feed to a rotary valve device comprising a single rotary valve or parallel rotary valves. The rotary valve device directs the feed to a conduit to adsorbent beds, which (viewed schematically, such as in the attendant drawings described herein) are located between the location of a first and second withdrawal stream. The first withdrawal stream is an extraction stream (which may comprise, by way of example, 99.7% PX, based on the amount of xylenes, and desorbent), and the second withdrawal stream is a raffinate stream (which comprises PX-depleted xylenes and desorbent). The conduits in fluid communication with the adsorption apparatus and the rotary valve(s) are shared with all of the feed and product streams, and, therefore, these lines must be flushed between the feed injection point and the extract withdrawal point in order to prevent contamination of the product. A standard unit has a first or primary flush which removes the majority of contaminants and a second or secondary flush which removes trace impurities before, preferably just before, the extract point.
The standard commercial simulated moving bed has only a single feed inlet. Various streams of different compositions may be blended together and fed to a single point in the Parex™ process. However, as indicated in U.S. Pat. No. 5,750,820 (see also U.S. Pat. No. 7,396,973), feeds, which are of substantially different composition, may be segregated from one another. For example, a feed, which is more highly concentrated in paraxylene, may be introduced upstream (in terms of the direction of the flow of circulating fluid) from a feed, which is less concentrated in paraxylene. An example of a feed, which is more highly concentrated in paraxylene, may be obtained from a selective toluene disproportionation unit. Such units may produce C8 aromatic mixtures having, for example, 85-90% paraxylene. Examples of feeds, which are less concentrated in paraxylene, may be obtained, for example, from a powerformer, isomerization unit or transalkylation unit. These units tend to produce equilibrium xylenes. These equilibrium xylenes may comprise a mixture of xylenes having, for example, about 23% paraxylene. The units which produce and recover xylenes, also tend to recover ethylbenzene. Ethylbenzene may be included as an impurity to the feed of a unit which produces paraxylene. The amount of ethylbenzene in the product recovered along with the equilibrium xylenes varies from process to process, depending on the type of process used to generate the equilibrium xylenes. For example, the proportion of ethylbenzene in the C8 aromatics produced and recovered in a reforming process may be different from the proportion of ethylbenzene produced and recovered in a transalkylation process.
As suggested in U.S. Pat. No. 5,750,820 (see also U.S. Pat. No. 7,396,973), the primary or first line flush may be used as a second feed point for the paraxylene concentrate, and the secondary flush may be used as the primary flushing stream.
There may be a problem with the above configuration when the standard Parex™ unit has the secondary flush located close to the extract withdrawal point. When the secondary flush is very close to the extract withdrawal point and concentrated paraxylene (having associated impurities) is flushed from the conduit in fluid communication with the rotary valve and the adsorption chamber, the configuration may be such that the point of the secondary flush is too close to the extract withdrawal point and the highest separation of the feed will not be realized.
This problem is addressed in U.S. Pat. No. 8,569,564. A solution is that the feed locations of both the primary flush (including concentrated paraxylene) and the secondary flush are modified to realize the full benefit of the feed configuration in U.S. Pat. No. 5,750,820. By moving the secondary flush further away from the extract, the material flushed from the conduit will be injected at a more efficient location. See U.S. Pat. No. 8,529,757. The problem and solution addressed in U.S. Pat. No. 8,569,564 are noted in the description of FIG. 1 in U.S. Pat. No. 8,529,757.
Facilities for producing C8 aromatics (paraxylene, metaxylene, orthoxylene, and ethylbenzene) often have at least one separation unit, such as a unit for conducting the Parex Process™, to separate paraxylene from the other components of the C8 aromatics. These facilities include petroleum refineries and petrochemical processing plants. These facilities may include a variety of units for producing C8 aromatics. Examples of such units for producing C8 aromatics include a selective toluene disproportionation unit, a powerformer (a type of a reforming unit), an isomerization unit and a transalkylation unit. Some units, such as powerformers, isomerization units and transalkylation units, tend to form equilibrium mixtures of C8 aromatics, for example, having 23% paraxylene and 77% of the sum of metaxylene and orthoxylene, based on the total of paraxylene, metaxylene, orthoxylene in the mixture. Other units, such as selective toluene disproportionation units, tend to form mixtures enhanced (in concentration) in paraxylene in comparison with an equilibrium mixture of C8 aromatics. For example, a selective toluene disproportion unit may produce a C8 aromatics mixture having 85 to 90% paraxylene and 10 to 15% of the sum of metaxylene, orthoxylene, and ethylbenzene, based on the total of paraxylene, metaxylene, orthoxylene, and ethylbenzene in the mixture.
Another unit, which tends to produce enhanced paraxylene, as opposed to equilibrium xylenes, is a selective toluene alkylation unit. The process conducted in the selective toluene alkylation unit involves alkylating toluene with an alkylating agent, such as methanol, with selective alkylation catalyst. The selective alkylation catalyst promotes the mono-alkylation of toluene with a methyl group in the para position to selectively produce paraxylene in preference to other isomers of xylene (MX and OX), as well as polyalkylated product (e.g., trimethylbenzenes). Such selective alkylation may be accomplished with the use of a catalyst comprising a medium pore size zeolite, such as ZSM-5. Such medium pore size zeolites have interior pore spaces, which allow access and egress of a molecule of the shape and size of paraxylene, yet resist the access and egress of a molecule of the shape and size of metaxylene, orthoxylene, and trimethylbenzene.
Facilities for producing C8 aromatics have varying capacities for producing C8 aromatic mixtures enhanced in paraxylene, for example, from selective toluene disproportionation units. Processes described above, for example, in U.S. Pat. No. 5,750,820, use an equilibrium xylene feed stream (e.g., having 23% of paraxylene and 77% of other xylenes, plus a varying amount of ethylbenzene) and replaces a primary flush stream with an extra feed comprising enhanced paraxylene feed (e.g., from a selective toluene disproportionation units). Ideally, the facility producing C8 aromatics in such a process would have a relatively large capacity to produce C8 aromatics enhanced in paraxylene (e.g., from a selective toluene disproportionation unit). Such a facility could produce enhanced paraxylene in sufficient amounts to (1) flush equilibrium xylene, comprising substantial amounts of contaminants, including metaxylene, orthoxylene, and ethylbenzene, from a conduit and (2) provide an additional source of paraxylene as a second feed step the overall separation process.
Facilities for producing C8 aromatic do not always have the capacity to produce enough enhanced paraxylene to best accommodate the dual feed process described in U.S. Pat. No. 5,750,820. For example, certain facilities may not produce enough enhanced paraxylene to even completely flush equilibrium xylene from conduits, much less provide a second feed of additional C8 aromatics to the separation process. Furthermore, facilities with relatively large capacities for producing enhanced paraxylene could benefit from using a minimal amount of enhanced paraxylene in a first conduit flushing medium and blending the remaining enhanced paraxylene with equilibrium xylene in the feed stream to the adsorption process. By introducing at least a portion of the enhanced paraxylene at a point upstream (relative to the flow of circulating fluid) from the first flush stage, a greater number of adsorbent beds are provided between the feed point and extract point. This greater number of catalyst beds may enhance the separation of paraxylene from other C8 aromatics.
The extract stream from the separation process may comprise desorbent, the desired paraxylene product and a very small amount of one or more unwanted C8 aromatics (i.e. metaxylene, orthoxylene, and ethylbenzene). The paraxylene product may be recovered by a distillation process. An extract stream or a recovered paraxylene product stream may be used as the first or primary flush stream to remove the residue of C8 aromatic feed remaining in the conduit. A desorbent stream may also be used as such a flush stream. However, there are problems with using recovered paraxylene, extract or desorbent in the primary flush stream. Recycling a portion of recovered paraxylene or extract stream to the primary flush stage limits product recovery. Introducing desorbent into a bed at the location of the primary flush may interfere with adsorption of paraxylene on the adsorbent. Minimizing the introduction of desorbent into the adsorption zone maximizes the adsorbent's capacity utilization. Furthermore, there are equipment and energy costs associated with routing any of (1) recovered paraxylene product, (2) extract and (3) desorbent to the primary flush stage of the recovery process. Therefore, a process that minimizes the use of recycled paraxylene, extract, or desorbent as a primary flush medium is desired.